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MECH270 Exam: Practice Q's
helpful long answer questions from MECH 270
51
Mechanical Engineering
Undergraduate 2
08/11/2017

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Term
Name the four methods of strengthening metals discussed in this course
Definition
Precipitate Strengthening, cold working, grain size strengthening, and solute hardening (interstitials)
Term
What is the key objective of strengthening on a microscopic scale
Definition
To reduce the mobility of dislocations.
Term
Explain ONE of the four methods of strengthening
Definition
Precipitate Strengthening: Involves the introduction of precipitates into a host lattice. These precipitates function as obstacles for dislocations. The applied stress must then be great enough for the dislocation to cut through the particle or bow between particles.

Cold Working: Involves putting plastic strain into a material such as rolling to introduce dislocations into the material. By adding dislocations you make it more difficult for dislocations to move once a stress is applied as they entangle with each other.

Grain Size Strengthening: Involves the reduction in size of the grains. As grain boundaries represent barriers to slip (they are not aligned perfectly and often are at great angles to the burgers vector) they prevent movement of dislocations from one grain to the next. By having more grain boundaries it is more difficult for dislocations to glide and requires more stress.

Solute Hardening: Involves the introduction of interstitials into the host lattice. These particles move to the dislocations and relieve stress. A small solute atom has an attractive interaction with the compressive strain field of a dislocation and a large solute atom has an attractive interaction with the tensile strain field of an edge dislocation. This attraction results in a lower energy state for the dislocation so when a stress is applied, it must be greater than the attraction in order for the dislocation to move.
Term
(d) Pairs of moving dislocation can interact with one another. One example result of this is the interaction of dislocation pinning; another example is dislocation repulsion. Choose one of these two example interaction and use a simple sketch to help explain why it occurs.
Definition
Dislocation Pinning: When the compressive and tensile strain fields of two dislocations line up perfectly, the effect is a cancelling out of all the strain or some of the strain.

Dislocation Repulsion: When compressive-compressive strain field’s line up there is a repulsion effect as no stress is relieved by coming together and it would put the dislocation at a higher energy state which is energetically unfavourable.

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Term
2. For any given metal, a certain critical (minimum) amount of cold working is required before recrystallization can occur (regardless of the recrystallization temperature employed).
Sketch and explain the relationship
Definition
[image]

The more strain placed into the material results in more dislocation and hence more places for nucleating to occur. Therefore the more plastic strain results in smaller grain sizes. The minimum strain is required because there needs to be a driving force for recrystallization to

occur. If there aren’t many dislocations then there would be no reason for new grain growth, ie. It wouldn’t be energetically favourable.
Term
What is the driving force for recovery
Definition
A reduction in free energy by movement of dislocations to grain boundaries.
Term
Draw a characteristic stress-strain curve for a typical semi-crystalline polymer (also called a plastic polymer). Label the yield and tensile strengths.
Definition
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Term
State where plastic deformation starts within the semi-crystalline polymer, explain why and describe what happens at the microscopic level during the first stage (only) of plastic deformation. Include as sketch or two, if needed.
Definition
The plastic deformation occurs in the crystalline regions where a shear force causes them to plastically deform and break into small pieces. The adjacent chains in lamella slide past one another breaking any of the weak secondary forces such as van der Waals.
Term
The properties of a specific polymer can vary hugely between different products. For example polyethylene can be used for shopping bags and for bullet-proof vests. State two key differences in the structure of the polyethylene contained in these two products, and explain why this leads to the differing properties.
Definition
Molecular densities increase TS as more chains have to unravel. Cross linking makes it harder for polymer chains to slide past one another.
Term
State two types of material that undergo a transition where they are brittle below some critical temperature, but are not brittle above that temperature. For steel, briefly explain why this abrupt transition in toughness occurs.
Definition
Steel (bcc) and semi-crystalline polymers.

This occurs in bcc steel as bcc has no close packed planes. However with the addition of temperature there is enough thermal energy to cause vibrations allowing bcc to behave as fcc, which has many close-packed planes required for slip.
Term
Explain the reason for the characteristic C-shaped curve of the TTT diagram.
Definition
There are two competing effect that form the C. The driving force is the undercooling; so for low temperatures there is large undercool resulting in large amounts of free energy. The system mobility is diffusion which is greater for higher temperatures. Therefore a medium temperature results in the quickest and optimal formation of the grains.
Term
Pearlite and bainite are both equilibrium two-phase microstructures comprised of ferrite and cementite. With reference to your answer to the shape of the TTT curve, explain what is different about the two microstructures.
Definition
The difference between the formations of the two is a result of the difference in temperature. For pearlite the temperature is higher so there is more diffusion and less undercooling resulting in more growth. Conversely for bainite, there is more under cool at a lower temperature so there is more of a driving factor resulting in many nucleation points and less growth. So bainite has much finer grains.
Term
All of the martensite transformation lines are horizontal on the TTT curve. Explain the reason for this shape.
Definition
The reason for the horizontal lines is because the transition is time independent. The transformation is only dependent on temperature as no diffusion can occur at such low temperatures.
Term
Explain how plastic deformation is produced in metals at the microscopic scale. Be specific about the microscopic feature(s) and mechanism(s) involved.
Definition
Plastic deformation is produced in metals by an applied stress causing the movement of dislocations along closed packed planes. The result is permanent deformation that does not revert when the load is removed.
Term
Compare the effect of temperature on the plastic deformation behaviour of BCC and FCC metals. Explain your answer.
Definition
BCC metals have no close packed planes, while FCC does. So at low temperatures FCC has much easier slip. However as the temperature increase there in more thermal energy and the BCC atoms vibrate enough to behave like closed packed planes so the slip is similar to FCC.
Term
Explain the mechanism of recovery, in terms of dislocation movement.
Definition
In recovery the dislocations move to the grain boundaries and compressive and tensile stain fields line up in configurations that are more energetically favourable.
Term
State what effect recovery has on the mechanical properties of a metal.
Definition
Recovery is the start of the process of annealing which reverses the effect of the cold work done on the material. Specifically it would increase the ductility of the metal and decrease the strength.
Term
The properties of composites depend on the properties of the two phases in the composite, matrix and reinforcement, and on the volume fraction of the two phases. The goal of making a composite is usually to improve the properties of the material relative to the matrix.
Give an example of a composite and state what property is improved.
Definition
Fibre-Glass: Increases yield strength and tensile strength.
Term
For aligned glass-fibres in an epoxy matrix, explain if you would expect a composite with long fibres to be stiffer or less stiff than a composite with short fibres (same volume fraction of fibres).
Definition
You would expect a composite with long fibres to be stiffer, that is have a larger young’s modulus when compared to a composite with short fibres. The reason is because composites with short fibres offer almost no resistance to applied stress and the matrix deforms around them without transferring any stress to the fibres.
Term
Aligned fibre composites show anisotropy in their behaviour. i) They show different strengths in tension and compression when loaded along the fibre direction. ii) They show different strengths when loaded in tension along the fibre direction compared to being loaded in tension perpendicular to the fibre direction. Explain why.
Definition
The reason composites show different strength in tension and compression when loaded along the fibre direction is because in compression long fibres buckle under the stress cause fracture in the fibres, while in tension there is no opportunity for buckling.
II) The reason for this is usually because the fibres are directional themselves. So their strength is in the direction they are oriented. In the case of the transverse direction the fibres have little strength and often the matrix itself is stronger. They can also not effectively distribute the applied stress.
Term
Explain why the strength of a ceramic is found to be less when tested in tension that in compression.
Definition
For compressive stresses, there is no stress amplification associated with any existent flaws. For this reason, brittle ceramics display much higher strengths in compression than in tension. This means that there are primarily cracks that would be open in tension rather than in compression.
Term
State the two key microstructural requirements for a polymer to behave as an elastomer and explain why they are required.
Definition
Elastomers must have an amorphous structure and be above the glass transition temperature. They must have an amorphous structure because the deformation of amorphous structures is purely elastic. They must also be above the glass transition temperature because the chains must be able to slide past each other in deformation, which requires chain rotation.
Term
Explain why an elastomer deformed at temperatures below some critical value (dependent on the polymer) will appear brittle.
Definition
This temperature, known as the glass transition temperature is the point at which chains of polymers can slide past one another. Below this temperature elastomers will appear brittle as during the deformation there will be no or little elastic deformation which is typically brought on by the unravelling of the many chains. Below Tg the chains are locked in place.
Term
Explain why, in general, it is easier to recycle thermoplastic polymers than thermosetting polymers.
Definition
t is easier to recycle thermoplastic polymers because they can be re-melted and cast into something new while thermosetting polymers are “stuck” in their solid phase. Heat will not melt thermosetting polymers.
Term
Precipitation hardening is only possible in alloys, for example some of those based on Al.
State the two features needed to produce precipitation hardening in a given alloy?
Definition
The two feature required are an interstitial that can be distributed uniformly throughout the host lattice and the interstitial must have decreasing solubility with decreasing temperature (so it comes out as a precipitate, not another phase).
Term
explain in terms of changes to the microstructure if the strength of a precipitation hardened Al alloy increase or decreases during over-aging.
Definition
The strength of the Al would decrease with over-aging, as the precipitates would have enough time to diffuse together and agglomerate. This would be more energetically favourable but the precipitates would be too big and not evenly distributed enough to prevent slip, thus causing a reduction in strength.
Term
On a common set of axes, draw typical engineering and true stress-strain curves for a metal. Label the key features on these curves.
Definition
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Term
On a separate set of axes from part (a), sketch engineering stress-strain curve for a metal with high toughness and one with low toughness. What is the one keep characteristic of these curves that indicates their relative level of toughness?
Definition
[image]

The area under a stress strain curve is an approximate value for the toughness of the material. The material with larger ductility also has much greater area and therefore a higher level of toughness.
Term
All BCC metals exhibit a ductile-to-brittle transition in their deformation behaviour. Define this concept and fully explain why it occurs for this particular crystal structure.
Definition
This is the ductile-brittle transition temperature. Occurs in BCC metals because they have one slip plane that is almost close-packed and with thermal energy it behaves that way, allowing for ductile deformation.
Term
Explain the concept of recovery. Be sure to mention the driving force for this phenomenon.
Definition
The driving force for recovery is free energy. The dislocations move to locations at the grain boundaries where they can be configured in energetically favourable positions.
Term
If a metal is cold worked and then held at high temperatures, does the amount of cold work affect the rate of grain growth that occurs? Explain your answer.
Definition
Yes, more cold work means more dislocations and more places for nucleation... therefore small grains.
Term
What is the driving force for diffusion?
Definition
Thermal Energy
Term
Are diffusion rates for interstitial atoms generally higher or lower than rate for substitutional atoms? Explain.
Definition
nterstitial atoms usually have a high diffusion rate. There is an energy barrier for the atoms to diffuse from its present site to a neighboring site. This barrier must be overcome by thermal energy – hence higher temperature means higher diffusion rate. Conversely for substitutional atoms to diffuse they need to overcome an energy barriers and have a vacancy beside them. Therefore the rate is slower than interstitial.
Term
What is the crystal structure of martensite? Explain how and why this structure can be produced in plain carbon steel.
Definition
Martensite is formed when austenite held at a temperature greater than the eutectic is quenched to room temperature. The quickness of the quench doesn’t allow for any of the carbon atoms to diffuse, and as a result they are “stuck” in place. As the carbon atoms are locked in place, there is a distortion in the crystal lattice structure. This results in the transformation of the FCC austenite to BCT (Body Centred Tetragonal) instead of BCC. The effect of this is a great increase in strength and a reduction in ductility as the distorted lattice structure does not allow for any slip (there are no close packed planes for dislocations).
Term
If a sample of martensite is re-heated to a moderate temperature eg. 600°C, the martensite progressively transforms into a new microstructure? What is the name of the composition of this new microstructure? Include a sketch of the microstructure.
Definition
If the Martensite is reheated (Tempered) to around 300 °C some of the carbon atoms can diffuse. This results in the formation of α and Fe3C with very fine grain sizes. Since the grain sizes are so small there are many grain boundaries so the tempered Martensite is almost as strong but is much more ductile due to α. However, if the martensite is over tempered then all or almost of the carbon diffuses out and the Martensite is converted to α and Fe3C. Since over tempering takes a long time, the α and Fe3C grains grow too big resulting in a reduction in strength
Term
What are the two stages in the formation of any new phase (ie. Phase transformation)? On a properly labelled set of axes, sketch a typical Avrami-type curve that governs the phase transformation process
Definition
Nucleation and grain growth.
[image]
Term
With the aid of a simple sketch, define the term branched polymer.
Definition
[image]

Branched polymers have side chains “branching” from the parent chain.
Term
For a typical semi-crystalline polymer, what three changes in stress-strain behaviour are caused by an increase in temperature?
Definition
Increasing the temperature results in a decrease in elastic modulus and tensile strength as well as an increase in ductility.
Term
For a linear polymer, such as polyethylene, describe three ways that the structure can be altered, without changing the chemical composition, in order to increase the polymer strength.
Definition
Without changing the chemical composition, the polymer’s strength could be increased by making the chains longer, allowing for slower cool so that there is a higher degree of crystallinity, pre-deform by drawing, and by annealing. Annealing promotes more inter-chain bonding.
Term
describe one type of point defect that can occur in a ceramic.
Definition
Frenkel Defect- Cation out of place
Shottky Defect- Paired set of anion and cation vacancies

Interstitial occurs when there is a cation out of place, however it must stay close to the vacancy. A vacancy occurs when there is a cation and anion pair missing. Again they two locations must stay close together and move as a pair.
Term
Explain the one key deformation property that enables amorphous ceramics (glasses) to be blown into intricate shapes. What minimum temperature is required for glass blowing?
Definition
They can be melted into a viscous liquid. The temperature required is 1600 degrees Celsius.
Term
define the term anisotropy in the context of fibre-reinforced composites.
Definition
Anisotropic means that the properties of a material are directional. In the case of fibre composites this occurs when the fibres are continuous and in one direction.
Term
During the process of annealing, explain how the grains actually grow.
Definition
An energy is associated with grain boundaries. As grains increase in size, the total boundary area decreases, yielding an attendant reduction in the total energy; this is the driving force for grain growth. Grain growth occurs by the migration of grain boundaries. Obviously, not all grains can enlarge, but large ones grow at the expense of small ones that shrink. Thus, the average grain size increases with time, and at any particular instant there will exist a range of grain sizes. Boundary motion is just the short-range diffusion of atoms from one side of the boundary to the other.
Term
Explain the origin of the ‘cup and cone’ characteristic failure in a metal.
Definition
The cup and cone failure in a metal is typical of ductile metals.
Term
Explain why the ductility of pure Al is significantly higher than the ductility of Al most alloys.
Definition
Most Al alloys are made through precipitate strengthening which increases the strength of the material, but decreases the ductility.
Term
At low stresses a value of n = 1, while at somewhat higher values of applied stress a value of n = 5 is often found for the steady state creep equation. Explain why the value of n changes.
Definition
The reason for the change is due to the nature of the creep at the different stresses. For low stresses, the creep is typically driven diffusion, where the applied stress isn’t enough for dislocation motion. Conversely at higher stress the creep is driven by the motion of the dislocations, leading to a faster creep rate and therefore a high value of n.
Term
Polymer X has a typical ‘brittle’ behaviour, while Polymer Y has a typical ‘plastic’ behaviour. Explain the differences between these polymers in terms of their bonding that might lead to this different behaviour.
Definition
Polymer X could have more crystalline regions that polymer Y. Additionally Polymer X could have cross links between adjacent chains or could be a network polymer, both which prevent the movement of chains past one another during deformation and this increase the strength at the cost of ductility.
Term
Explain what is typically different about the point defects in ceramics compared to those found in metals.
Definition
Point defects in ceramics must maintain the charge neutrality.
Term
Ionic ceramics can have a large range of structures; list the two characteristics that control the crystal structure of a given ionic ceramic.
Definition
Two characteristics of the component ions in crystalline ceramic materials influence the crystal structure: the magnitude of the electrical charge on each of the component ions, and the relative sizes of the cations and anions.
Term
Creep can occur in a pure metal sample by a mechanism of self-diffusion. At an atomic level, what are the two necessary conditions for self-diffusion?
Definition
There needs to be enough thermal energy in order to overcome the energy barrier for the jump and there needs to be a vacancy next to the atom.
Term
Certain ionically-bonded ceramics are comprised of BCC or FCC crystal structures, which means they theoretically contain the same sets of slip systems as BCC or FCC metals. However, these ceramics are not able to produce plastic deformation. Explain why.
Definition
Ceramics are not able to produce plastic deformation as any motion of point defects must maintain the charge neutrality. Additionally ceramics typically fail before plastic deformation due to crack propagation.
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